1. Field of the Invention
The present invention, in at least certain embodiments, is directed to methods for efficiently recovering beads or spheres from a dual density drilling fluid; to systems useful in such recovery; and to dual gradient drilling systems and processes that use such bead or sphere recovery methods.
2. Description of Related Art
The prior art discloses a variety of systems for providing drilling fluids (often referred to as “drilling mud”) for oil and gas drilling applications and methods and apparatus for varying the density of mud in deep water oil and gas drilling operations. In many such methods drilling mud drives drill bits, maintains hydrostatic pressure, and carries away particulate matter, debris, and drilled cuttings. In many methods drilling mud is pumped down the drill pipe and provides the fluid driving force for a drill bit and then it flows back up from the bit along the periphery of the drill pipe in an annulus between the drill pipe and a tubular or an open hole's interior for removing the particles drilled away by the drill bit. Mud returning to the surface is cleaned to remove the particles, debris, drilled cuttings, etc. and recycled down into the wellbore.
In many prior art methods, density of the drilling mud is monitored and controlled to maximize the efficiency of the drilling operation and to maintain a desired hydrostatic pressure. A well is drilled in many typical operations using a drill bit mounted on the end of a drill stem inserted down the drill pipe. Mud is pumped down the drill pipe and through the drill bit to drive the bit. A gas flow can also pumped and/or other additives are also pumped into the drill pipe to control the density of the mud. Mud passes through the drill bit and flows upwardly along the drill string inside the open hole and casing, carrying the drilled cuttings etc. to the surface. U.S. Pat. No. 5,873,420 discloses an air and mud control system for underbalanced drilling which provides, among other things, for a gas flow in the tubing for mixing the gas with the mud in a desired ratio so that mud density is reduced to permit enhanced drilling rates by maintaining the well in an underbalanced condition.
Formation pressure on earth formations increases as a function of depth due to the weight of the overburden on particular strata. This weight increases with depth so the prevailing or quiescent bottom hole pressure is increased in a generally linear curve with respect to depth. As the well depth is doubled, the pressure is also doubled. When drilling in deep water or ultra deep water this is further complicated because of the pressure on the sea floor by the water above it. High-pressure conditions exist at the beginning of the hole and increase as the well is drilled. A balance must be maintained between the mud density and pressure and the hole pressure or the pressure in the hole will force material back into the well bore and cause what is commonly known as a blowout in which gases in the well bore flow out of the formation into the well bore and bubble upward. When the standing column of drilling fluid is equal to or greater than the pressure at the depth of the borehole the conditions leading to a blowout are minimized. When the mud density is insufficient, the gases or fluids in the borehole can cause the mud to decrease in density and become so light that a blowout occurs which can bring drilling operations to a halt and cause significant damage and injury. Usually blowout preventers or BOP's are installed at the ocean floor to minimize a blowout from an out-of-balance well. One primary method for minimizing blowout is the proper balancing of the drilling mud density to maintain the well in balance at all times. While BOP's can contain a blowout and minimize the damage to personnel and the environment, the well is usually lost once a blowout occurs, even if contained. Proper mud control techniques can reduce the risk of a blowout and obviate the need to contain a blowout once it occurs. In certain methods, to maintain a safe margin, the column of drilling mud in the annular space around the drill stem is of sufficient weight and density to produce a high enough pressure to limit risk to near zero in normal drilling conditions, but this can slow the drilling process. Underbalanced drilling is sued in some prior art methods to increase the drilling rate.
The need to provide a high density mud in a well bore that starts several thousand feet below sea level in deep water or ultra deep water drilling can present a variety of problems. Pressure at the beginning of the hole is equal to the hydrostatic pressure of the seawater above it, but the mud must travel from the sea surface to the sea floor before its density is useful. To maintain mud density at or near seawater density (or 8.6 PPG) when above the borehole and at a heavier density from the seabed down into the well is desirable. Pumps have been employed in certain prior art methods near the seabed for pumping out the returning mud and cuttings from the seabed above the BOP's and to the surface using a return line that is separate from the typical subsea riser system, a system which is expensive to install, requiring separate lines, expensive to maintain and very expensive to run.
In typical offshore drilling, a riser extends from the sea floor to a drill ship and drilling fluid is circulated down the drill stem and up the borehole annulus, the casing set in the borehole, and the riser, back to the drill ship. The drilling fluid performs several functions, including well control. The weight or density of the drilling fluid is selected so as to maintain well bore annulus pressure above formation pore pressure, so that the well does not “kick”, and below fracture pressure, so that the fluid does not hydraulically fracture the formation and cause lost circulation. In deep water, the pore pressure and fracture pressure gradients are typically close together. In order to avoid lost circulation or a kick, it is necessary to maintain the drilling fluid pressure between the pore pressure gradient and the fracture pressure gradient.
The drilling fluid hydrostatic pressure gradient in conventional riser drilling is a straight line extending from the surface. This hydrostatic pressure gradient line traverses across the pore pressure gradient and fracture pressure gradient over a short vertical distance, which can result in having to set numerous casing strings. The setting of casing strings is expensive in terms of time and equipment. Various prior art systems—called dual gradient drilling systems—disclose attempts to decouple the hydrostatic head of the drilling fluid in the riser from the effective and useful hydrostatic head in the well bore. In dual gradient systems, the hydrostatic pressure in the annulus at the mud line is equal to the pressure due to the depth of the seawater and the pressure on the borehole is equal to the drilling fluid hydrostatic pressure. The combination of the seawater gradient at the mud line and drilling fluid gradient in the well bore results in greater depth for each casing string and a reduction of the total number of casing strings required to achieve any particular bore hole depth.
Various methods in the prior art have been proposed to produce an efficient and effective dual gradient system. In one method drilling fluid returns are continuously dumped at the sea floor. This is not safe, environmentally practical or economically viable. In another method, gas lift is used involving injecting a gas such as nitrogen into the riser. This requires no major subsea mechanical equipment, but it has some limitations. Since gas is compressible, the depth at which it may be utilized is limited and extensive surface equipment may be required. Also, because the gas expands as the drilling fluid reaches the surface, surface flow rates can be excessive.
Another prior art attempt to create an effective dual gradient system is pumping the drilling fluid from the underwater wellhead back to the surface. Several pumping systems have been suggested, including jet style pumps, positive displacement pumps, and centrifugal pumps. Sea floor pump systems provide the flexibility needed to handle drilling situations, but they have the disadvantage of high cost and reliability problems associated with keeping complex pumping systems operating reliably on the sea floor.
U.S. Pat. No. 6,536,540 issued Mar. 25, 2003 and U.S. Patent Application 20030070840 published Apr. 17, 2003 disclose, among other things, methods and apparatus for controlling drilling mud density at or near the sea bed of wells in deep water and ultra deep-water applications. By combining the appropriate quantities of drilling mud with a base fluid of lesser density, a riser mud density at or near the density of seawater may be achieved. No additional hardware is required below the surface. The riser charging lines are used to inject the low-density base fluid at or near the BOP stack on the seabed. The cuttings are brought to the surface with the diluted mud and separated in the usual manner. The diluted mud is then passed through a centrifuge system to separate the heavier drilling mud from the lighter base fluid.
Another prior art method employs the injection of low-density particles such as glass beads into the returning fluid in the riser above the sea floor to reduce the density of the returning mud as it is brought to the surface. Glass beads are injected above the BOP stack. U.S. Pat. No. 6,530,437 discloses such methods in which a multi-gradient system for drilling a well bore from a surface location into a seabed includes an injector for injecting buoyant substantially incompressible, e.g. glass beads, articles into a column of drilling fluid associated with the well bore. In one such method, the substantially incompressible articles are hollow substantially spherical bodies. All patents and applications referred to herein are incorporated fully herein for all purposes.